For a digital camera or a video camera, which is one type of image-taking unit, a charge coupled device (CCD) image sensor or a complementary metal-oxide semiconductor (CMOS) image sensor is widely used as an imaging device. An imaging device includes an imaging element and a color filter array. The imaging element includes a plurality of photoelectric converters two-dimensionally arranged at predetermined intervals (hereinafter referred to as a “pixel pitch”). The color filter array is disposed on a light input surface of the imaging element. The imaging device obtains color image data by discretely sampling subject light coming in through a taking lens, with the imaging element and the color filter array.
The imaging device has a resolution limit (Nyquist frequency) determined by the pixel pitch of the imaging element and a color array pitch of the color filter array. For this reason, when subject light having a high-frequency component of the Nyquist frequency or more enters the imaging device, the high-frequency component of the Nyquist frequency or more becomes an aliasing component in real space. This generates a stripes-like pattern (moire) in which color and luminance of image data periodically vary. Usually, an optical low pass filter (OLPF; liquid-crystal low pass filter) with the Nyquist frequency set to a cut-off frequency is provided between the taking lens and the imaging element, thereby removing the high-frequency component of the light entering the imaging element.
In a camera enabling both of shooting a still image and shooting a moving image, a reading pitch in a still image mode and a reading pitch in a moving image mode are different, and the respective Nyquist frequencies are therefore different as well. Usually, the reading pitch in the moving image mode is larger than the reading pitch in the still image mode, while the Nyquist frequency in the moving image mode is less than the Nyquist frequency in the still image mode. This makes it easier to cause moire from a low-frequency domain in the moving image mode, than in the still image mode. However, in the past, a high priority is given to resolution of the still image, and therefore the cut-off frequency of the OLPF is set to the Nyquist frequency in the still image mode. In other words, an OLPF optimal for both of these modes has not been provided.
For such an issue, there is proposed to provide OLPFs having an optimal cut-off frequency for respective modes to perform switching between the OLFPs in accordance with the mode, as discussed in PTL 1. There is also proposed an OLPF in which a twisted nematic (TN) liquid crystal is interposed between a pair of liquid crystal plates, and the TN liquid crystal is driven to be turned on and off to change a ps separation width of transmitted light into two types, as discussed in PTLs 2 and 3. Note that it is possible to change the cut-off frequency by changing the ps separation width.